The present invention relates to a method and an apparatus for venous compression plethysmography.
Venous compression plethysmography is a procedure which has been known for some time now and which is used for determining microvascular parameters in the extremities, such as the venous capacity, the venous elasticity, the venous outflow rate, the arterial blood flow and the capillary filtration rate. In general, venous compression plethysmography allows qualitative and quantitative statements to be made concerning the state and function of the microvascular circulation in an extremity of a patient.
Venous compression plethysmography can be carried out in a very wide variety of ways, for example as water plethysmography, air plethysmography, impedance plethysmography, capacitance plethysmography, induction plethysmography or strain-gauge plethysmography. These procedures make use of different physical phenomena for determining the state of the blood vessels in a body part. The present invention relates to strain-gauge plethysmography and to compression plethysmography by means of direct induction-based measurement of circumferential change.
In strain-gauge plethysmography, an expandable strain-gauge is laid around the body part which is to be examined, for example an arm or a leg. The venous return of the blood is then obstructed in this body part using an inflatable cuff arranged nearer to the heart. The blood congestion leads to a change in the circumference of the body part in question, which in turn leads to expansion of the strain-gauge. From the expansion of the strain-gauge, which depends on the pressure applied to the cuff, it is possible to draw conclusions regarding characteristics or changes in the blood vessels. This evaluation of the expansion as a function of the induced congestion is based on known procedures.
In strain-gauge plethysmography today, it is normal to use a strain-gauge consisting of an expandable silicone tubing filled with mercury. In the event of expansion of the body part around which the strain-gauge is laid, the silicone tubing expands and deforms the mercury column located therein. As a result, the electrical resistance of the mercury column changes. This change in resistance is measured and from this it is then possible to draw conclusions regarding the expansion of the silicone tubing and, consequently, of the body part. This conclusion necessarily assumes knowledge of the relationship between the change in resistance and the strain-gauge expansion. Apparatuses for strain-gauge plethysmography must therefore be calibrated prior to their use. This calibration must be carried out virtually before each single examination of a body part, since the ratio of the resistance of the mercury column to the expansion of the strain-gauge depends on a large number of parameters, such as the ambient temperature and the patient""s body temperature, the initial stress of the silicone tubing and the circumference of the body part examined.
The calibration is carried out by expanding the strain-gauge in a defined manner, while it is fastened in the examination position, and by measuring the change in resistance which occurs. In this case, several successive defined expansions of the strain-gauge are normally carried out, and the associated changes in resistance measured. A conversion factor is then calculated from this, assuming a linear relationship or other relationship of the measurement parameters. For calibration purposes, a calibration apparatus is arranged on the strain-gauge, and in conventional equipment this calibration apparatus generally consists of a knurled screw for expanding the strain-gauge. During calibration, the operator, for example a doctor or a nurse, then successively adjusts the length of the strain-gauge manually.
With this manual changing of the expansion, the operator is continuously exerting disturbing forces on the mostly very light measuring system, so that, for an exact measurement, it is necessary to wait for the relaxation of the system again and again. The relaxation times of the system can in these cases be very long and very different. An additional expansion of the silicone tubing, for example, can relax relatively quickly, whilst an impression in the tissue of the body part to be examined requires very much more time to return to the initial state. This leads to considerable time losses in the examination. Moreover, in the case of manual contact with the measurement device which bears on the body part, there is the risk of the strain-gauge being shifted on the patient""s skin. This can lead to errors in the calibration and, thus, to inaccuracies in the evaluation of the measured values. A particular disadvantage is that it very much depends on the skill of the operator as to whether the measurement can be carried out quickly and reliably. The measurement thus loses some of its reliability as well as its reproducibility. For the measurement error caused by the operator to remain unchanged during a series of measurements, it would in fact be necessary for the same operator to carry out all the measurements. This is of course not really possible in prolonged series of tests and research projects.
The object of the invention is therefore to make available a method and an apparatus for strain-gauge plethysmography, which method is quick and easy to use but at the same time reliably provides accurate measured values.
According to one aspect of the invention there is provided a method for venous compression plethysmography, in which an extremity is surrounded by a cuff, whose internal diameter I can be varied, in such a way that an obstruction of the blood outflow can be generated in those veins of the extremity which are situated remote from the heart in isolation to the cuff, where a strain-gauge is arranged on the extremity in order to encircle the latter, at a point remote from the hear in relation to the cuff, in such a way that a tissue distension of the extremity, occurring as a result of an obstruction of the blood outflow, causes an expansion xcex94D of the strain-gauge, where the expansion xcex94D of the strain gauge, as a function of a measurement of the change xcex94I of the internal diameter I of the cuff, is detected by determining the change xcex94M of a measurement parameter M, and where the relationship between the strain-gauge expansion xcex94D and the measurement parameter change xcex94M is determined by calibration with the aid of a calibration apparatus connected to the strain-gauge, by determining at least one measurement parameter change xcex94M12 for a defined expansion xcex94D12, characterized in that the defined expansion xcex94D for the calibration is generated by an adjustment mechanism in the calibration apparatus without an operator touching the strain-gauge or the calibration apparatus.
According to a further aspect of the invention, there is provided an apparatus for venous compression plethsymography with a cuff, whose internal diameter I can be varied, and which is suitable for encircling an extremity, with a measurement device arranged distally thereof, characterized in that the measurement device consists of a first area which is laid around the extremity and has an essentially low-expansion, dimensionally non-stable force transmission element which is guided round the circumference of an essentially band-like support belt, of a second area which is in operational communication with the two ends of the low-expansion, dimensionally non-stable force transmission element in such a way that a circumferential linear change of the extremity, by means of the low-expansion, dimensionally non-stable force transmission element, is detected by a measurement apparatus, where one end of the low-expansion, dimensionally non-stable force transmission element is in contact with the measurement apparatus, and the other end of the low-expansion, dimensionally non-stable force transmission element is secured on an adjustable bearing arrangement, or in that the measurement device consists of a first area which is laid around the extremity and has an essentially expandable, dimensionally non-stable force transmission element which is guided round the circumference of an essentially band-like support belt, of a second area which is in operational communication with the two ends of the expandable, dimensionally non-stable force transmission element in such a way that a circumferential linear change of the extremity, by means of the expandable, dimensionally non-stable force transmission element, is detected by a measurement apparatus, where one end of the expandable, dimensionally non-stable force transmission element is in contact with the measurement apparatus, and the other end of the expandable, dimensionally non-stable force transmission element is secured on an adjustable bearing arrangement.
In the context of the present invention, a method and an apparatus are made available for venous compression plethysmography using a strain-gauge, where the strain-gauge is calibrated without being touched by an operator. The calibration is preferably done automotively using an electric motor, a pneumatic mechanism or a spring mechanism, which motor or mechanism provides for the adjustment of a knurled screw or other advancing device, so that the desired defined expansion can be set. Use of a microprocessor for controlling the calibration device is particularly preferred. In this way, the calibration of the expansion measurement can be undertaken without direct contact between the operator and the strain-gauge or calibration device.